Fmcw distance measuring method and devices

ABSTRACT

A method of measuring the distance uses a terminal A and a terminal B, each terminal comprising an FMCW generator for generating a signal having a predetermined frequency to time function. Terminal A transmits a transmit signal in transmit periods, which alternate with mute periods without transmission. Terminal B shifts the received signal in time from a transmit period to a mute period, for example by generating a local periodic FMCW signal, determining an adaptation of the timing of that local periodic FMCW that is needed to bring it into a predetermined timing relation to the received signal and applying the adaptation in the mute period. Terminal B transmits the shifted signal, which is received by the receiver of terminal A. Terminal A measures the frequency offset between its own transmit signal and the signal received from terminal B and calculates the distance between terminal A and terminal B from that measured frequency offset, taking into account said predetermined time offset performed in terminal B. Preferably, the transmit signal is constituted by alternating a transmit period, including n cycles of the signal generated by the FMCW generator, and a mute period corresponding with n cycles of that signal, n being at least 1.

FIELD OF THE INVENTION

The present invention relates to measuring the distance betweenterminals, a terminal for performing such measurements and a systemcomprising such terminals. More in particular, the present inventionrelates to a method for measuring the distance between a tag A and a tagB, and to tags suitable for carrying out the method.

BACKGROUND

British Patent GB 1 219 410 discloses an FMCW (frequency modulationcontinuous wave) secondary radar system in which a transponder (onanother aircraft or on the ground) returns the incoming signal with afrequency-shifted carrier, and the frequency modulation on the output ofthe detector is discriminated to give velocity and/or distance. Thefrequency-shift at the transponder may be height-coded and the systemmay be utilized for anti-collision protection. Timesharing may beadopted to facilitate multistation operation. To avoid synchronousmodulation very low-frequency noise modulation may be added. Directionalemission may be utilized. The frequency modulation may be sinusoidal orsawtooth.

British Patent Application GB 2 243 739 discloses a technique for shortrange distance measurement using sub-microwave radio frequency signalsin which the distance between a measurement unit and a co-operativeactive target unit is determined as follows: The measurement unittransmits a carrier signal which is frequency modulated with a lowfrequency modulation signal. This is received by the target unit whichapplies a small frequency shift to the signal and re-transmits it. Themeasurement unit receives the frequency shifted signal and mixes it withits transmitted signal to form a further frequency modulated signal inwhich the carrier frequency is equal to the frequency shift applied bythe target unit and the modulation signal is at the frequency of thetransmitter modulation and takes a form dependent on the round tripdelay between measurement and target units and hence distance. Thenature of the dependence is determined by the modulation waveform. Ifthis is sinusoidal, the modulation amplitude is directly proportional todistance. By measuring this amplitude the distance can thus bedetermined. As a result of the frequency shift in the target unit,forward and return paths can be separated and therefore operation atsub-microwave radio frequencies is possible.

U.S. Pat. No. 5,442,357 discloses a control circuit of a radartransponder which produces pulses when the radar transponder detects aradio wave from a search radar. The pulses are sent to a sweep signalgenerator in which sawtooth waves are obtained through a first constantcurrent circuit for linearizing the voltage-to-time characteristic in afalling portion of a sawtooth. The sweep signal generator further has asecond constant current circuit for linearizing the voltage-to-timecharacteristic in a rising portion of a sawtooth.

These publications utilize techniques which are known from FMCW radar,where a stable frequency signal, frequency modulated by e.g. a saw toothor (other) triangular modulation signal, is transmitted. A signalreflected from a target object is mixed (multiplied) with thetransmitted signal. The reflected signal received t_(e) millisecondslater is mixed with a portion of the transmitted signal to produce abeat signal at a frequency f_(b), which is proportional to theround-trip time t_(e).

Conventionally FMCW uses objects which simply reflect incoming FMCWsignals, received from an FMCW transceiver, back to their location oforigin. Disadvantageous is the signal attenuation ratio due to apropagation loss ratio of 1/d² over the path d between the transmitterand the object. This attenuation ration applies for both the path fromthe transceiver to the object and, backwards, the (reflection) path fromthe object to the transceiver, resulting in an attenuation ratio of1/d⁴. By using objects which are provided with amplifiers, thisattenuation could be compensated.

In practice, however, due to among others problems arising from thelarge ratio between the power of the signal received by the object andthe power with which the transponder transmits the amplified signal backto the radar transceiver, simply amplifying and sending back thereceived signal without any processing will not work. Such processingcould comprise sending back the signal in another frequency band, notinterfering with the frequency band of the received FMCW signal.However, this increases frequency usage.

SUMMARY

It is an object to provide accurate measurements of distance betweenterminals, wherein interference between transmissions from the terminalsis avoided.

A method according to claim 1 is provided. Herein different periods ortime slots are used for sending from a radar transceiver terminal to asecond terminal and for sending from the terminal to the radartransceiver terminal. A time shifted signal is transmitted in a muteperiod of the radar transceiver terminal. A terminal which is arrangedto transmit a “reflected” signal within the mute period may include alocal generator for generating a recovered and amplified reflectionsignal towards the radar transceiver.

In practical situations all kinds of unforeseen reflections may occur.For that reason the time offset may e.g. correspond to the time neededfor transmission over a distance offset of e.g. 10 meters or more. Thetime offset can easily be compensated in the distance calculation.

It is preferred that the transmit signal is constituted by alternating atransmit period including n cycles of the signal generated by the FMCWgenerator, n being at least 1. The mute period preferably has a durationof at least one cycle. In an embodiment the duration of the mute periodalso corresponds with n cycles of the signal.

In an embodiment, wherein the shifted signal is generated by generatinga periodic signal in the second terminal (B), the periodic signal havingsaid predetermined frequency to time relation in successive cycles,detecting a timing difference between the periodic signal and thereceived signal in the transmit period in the second terminal (B),determining an adaption of a timing of the cycles of the periodic signalfrom the detected timing difference, and deriving the shifted signalfrom the periodic signal in the mute period, with said adaptation. Thetiming of the periodic signal may be adapted for example so that cyclesof the received signal and the periodic signal during the transmitperiod start in a predetermined time relation with each other, theresulting adaptation being used also in the mute period. As anotherexample, the adaptation that is needed to realize such a time relationmay be determined from the detection in the transmit period and appliedin the mute period. In addition to the detection dependent adaptation anadditional time shift may even be applied in at least the mute period,the additional time shift having a predetermined length that is known interminal A, or a length that can be determined in terminal A. Terminal Ais able to compensate the distance measurements for any time shift thatis known to be applied in terminal B.

To be able to accurately measure the distance d between the radartransceiver terminal and the second terminal, when there is an unknowndrift of the local FMCW generator (or oscillator), a synchronizationstep may be performed. This synchronization may be performed in aniterative process, achieving that, after synchronization, the local FMCWgenerator runs synchronously, having a predetermined time offset withthe FMCW modulation of the received signal.

The FMCW radar transceiver terminal and the second terminal(s) may betags, may be used in a symmetrical way, i.e. in a mutually equivalentway to another. Such terminals may be incorporated in a network whereineach of the terminals may communicate with each other while, besides,they will be able to measure accurately their mutual distances and,accordingly, their positions relative to another.

The terminals may be tags worn by persons, which could make such systemsuitable for use in e.g. military operations, fire brigades, police etc.The tag may be a mobile phone, a wristwatch etc.

In this specification and claims the notion FMCW (=Frequency ModulatedContinuous Wave) is deemed to include PMCW (=Phase Modulated ContinuousWave). Also possible different signals having a predetermined frequencyto time function are deemed to be included in the notion FMCW.

The frequency to time function of the FMCW signal may show a saw toothshape (having one slope). In an embodiment a triangular (“zigzag”) shapemay be used, having two similar slopes in opposite directions, whichoffers the opportunity to compensate distance calculations for frequencydrift.

In addition a frequency offset may be applied. A frequency offset is ofe.g. 2 MHz may be used, which can be easily detected. In the distancecalculation the influence of this frequency offset is compensated.

The present invention additionally provides a computer program productfor carrying out the method as defined above. A computer program productmay comprise a set of computer executable instructions stored on a datacarrier, such as a CD or a DVD. The set of computer executableinstructions, which allow a programmable computer to carry out themethod as defined above, may also be available for downloading from aremote server, for example via the Internet

BRIEF DESCRIPTION OF THE DRAWING

These and other objects and advantages will become apparent from adescription of exemplary embodiments, using the following figures.

FIG. 1 illustrates two terminals and a synchronization action;

FIG. 2 shows an exemplary embodiment of a terminal includingsynchronization means;

FIG. 3 illustrates a preferred signal transmission scenario;

FIG. 4 illustrates the use of a double sloped triangular FMCW signal.

FIG. 5 shows an exemplary embodiment of a terminal set

FIGS. 5 a-i show beat frequency from synchronisation amplitudes as afunction of the beat frequency in various cases;

FIG. 6 shows a correlator module.

FIG. 7 shows a terminal

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows two terminals A and B, each including an antenna. An FMCWgenerator is provided for generating a signal having a predeterminedfrequency to time function, represented in FIG. 1 by an FMCW signalwhich linearly sweeps between a frequency f_(a) and a frequency f_(b)under control of an FMCW generator. Each terminal A, B comprises atransmitter for transmitting a transmit signal comprising the FMCWsignal. The following steps are performed:

-   a. terminal A sends an FMCW signal;-   b. the signal is received by terminal B;-   c. terminal B (continuously) generates a local FMCW signal;-   d. terminal B synchronizes the local signal to the received signal    by shifting either the received signal or—preferably—the local    signal in time such that the local signal has a predetermined time    offset compared with the received signal;-   e. the local signal, i.e. the signal which was locally generated (c)    in terminal B and synchronized (d) relative to the signal    received (b) by terminal B from terminal A, is transmitted by    terminal B after synchronization (d);-   f. this synchronized signal, transmitted by terminal B (e), is    received by terminal A;-   g. the signal received by terminal A (f) is compared then with (a    representative of) the originally sent signal and the distance    between terminals A and B is calculated from (measured) Δf₂, i.e.    the distance d between terminals A and B is

d[m]=Δt ₂ [sec]*c [speed of light in msec⁻¹]  [1]

-   -   As—common in FMCW based devices—the value of Δf₂ is measured in        terminal A rather than the value of Δt₂, d is calculated by

d[m]=Δf ₂ [sec⁻¹]*(df/dt)⁻¹ [sec² ]*c [msec⁻]  [2],

-   -   where df/dt represents the frequency sweep of the (saw tooth        shaped) FMCW signal, i.e. the slope of the graphical        representation of it, as shown in FIG. 1. When, in terminal B,        the received FMCW signal is synchronized to the locally        generated FMCW signal, viz. by shifting the received FMCW signal        towards the “position” of the local FMCW signal, d (in formula        [2]) represents the single distance between terminals A and B.        Inversely, when—as will be preferred—in terminal B the locally        generated FMCW signal is shifted to become synchronous with the        received signal, d (in formula [2]) represents the distance        A−B−A; in other words d represents twice the distance A−B.

In addition a frequency offset may be applied. In an embodiment thefrequency offset is set to e.g. 2 MHz. This (predetermined) value isalso discounted in the calculation of the distance between terminals Aand B. Such intentional deviation from zero has the advantage that theelectronic circuitry works better when the frequency offset is unequalto zero.

FIG. 2 shows an exemplary embodiment of terminal B (terminals A and B,however, may have similar configurations) including synchronizationmeans. An FMCW signal is received via an antenna 1 of terminal B. Agenerator 2 within terminal B generates a local FMCW signal. Thereceived and locally generated signals are mixed in a mixer 3. Thereceived signal is applied to the mixer 3 via a switch 4. Both thegenerator 2 and the switch 4 are controlled by a controller 5. Thecontroller 5 detects the frequency difference Δf₁ or time difference Δt₁between the received and locally generated signals. in an iterativeprocess the controller 5 shifts the received signal in frequency and/orin time such that the local signal has a predetermined frequency offsetand/or time offset compared with the received signal, which offset maybe equal to zero of have another predetermined value. As soon as bothFMCW signal thus have been synchronized the switch 4 may be turned over,causing that the synchronized FMCW signal is fed to the antenna 1, viaan amplifier 6.

FIG. 3 shows that the terminals A and B are arranged so that thetransmit signal of each terminal is constituted by an alternatingtransmit period, including n cycles of the signal generated by the FMCWgenerator of terminal A and B respectively, and a mute periodcorresponding with n cycles of that signal, n being at least 1. In FIG.3 n=1. So, terminal A and terminal B send their FMCW cycles (n=1 in FIG.3) in an alternately way. In this way each terminal A and B is able toreceive n cycles, synchronize it and switch over (see switch 4 in FIG.4) transmit the synchronized FMCW cycle(s), without interference.

Transmission in the mute period has the advantage that the samefrequencies can be used for the transmit signal from terminal A and thereturned signal from terminal B, which reduces bandwidth use.Interference is reduced.

In an embodiment the number of cycles in the transmit period is notequal to the number of cycles in the mute period. A mute period of onecycle may be used for example, as one cycle suffices to determine thedistance. In this case a plurality of transmit periods may be used toimprove the synchronization before transmission from terminal B.

The predetermined frequency to time function of the FMCW signal may showa saw tooth shape (having one slope and a substantially instantaneousswitch back to the starting frequency from the end of the slope), asshown in FIGS. 1-3. However, it is preferred that said predeterminedfrequency to time function has a triangular (“zigzag”) shape, having twoopposite slopes, which offers the opportunity to compensate distancecalculations for frequency drift, as can be seen in FIG. 4.

FIG. 4 shows one cycle of a double sloped triangular FMCW signal,i.e.—from top to bottom—a received FMCW signal, a locally generated FMCWsignal and a locally generated FMCW signal having a frequency driftΔf_(d). The distance d is calculated from the values of Δf_(a) andΔf_(b): d=Δf_(a)−Δf_(b). Due to the frequency drift Δf_(d) Δf_(a) mayhave become Δf_(a)′ and Δf_(b)Δf_(b)′. As can be seen in FIG. 4, thefrequency drift Δf_(d), however, does not affect the value of d, as devalue of Δf_(a)−Δf_(b) is equal to the value of Δf_(a)′−ΔF_(b)′. Due tothis a doubled sloped triangular FMCW form is advantageous over a singlesloped, saw tooth shaped FMCW signal form.

Terminals A and B, may have similar configurations. Both may comprise anFMCW generator 2 and optionally an amplifier 6 that are used to transmitan FMCW signal. Controller 5 of terminal B is configured to switchbetween a sync mode wherein terminal B uses the local FMCW signal to mixdown the received signal and a transmit mode wherein the local FMCWsignal is used to transmit the FMCW signal back to terminal B. Mixingdown with the local FMCW signal reduces the frequency range of the mixeddown signal. The FMCW signal may have a sweep of between 10-100 Mhz forexample (e.g. 80 Mhz). Terminal B may comprise a filter in mixer 3, asis conventional, to filter the mixed down signal before supplying it tocontroller 5. The filter may have a bandwidth between 1-10 Mhz(preferably 1-2 Mhz, 1.5 Mhz for example), which is lower than the sweepbandwidth (e.g. at least a factor 10 lower). By mixing with the localFMCW signal the meaningful frequency range mixed signal can be keptwithin the filter bandwidth. Mixer 3 may be a quadrature mixer, whichproduces in phase and quadrature phase mixing results. Quadrature mixersare known per se: they may involve mixing the received signal with an inphase version of the local FMCW signal and a quadrature phase FMCWsignal. When a quadrature mixer is used, two filters may be used, forthe respective signal components. The filter or filters may also beconsidered as part of controller 5.

Controller 5 of terminal B is configured to control FMCW generator 2 toadapt at least the timing of the local FMCW signal in the sync mode, inorder to synchronize with the received signal. Various controlstrategies are possible. In an embodiment, controller 5 aims to maintaina predetermined time delay and frequency shift between the sweep of thereceived signal and the local FMCW signal. In this embodiment,triangular sweeps such as shown in FIG. 4 are used, or at least partlytriangular sweeps, which comprise a part wherein the FMCW frequencyfirst changes in a first direction at a frequency sweep rate andsubsequently changes in a second direction, opposite to the firstdirection, at said frequency sweep rate. The uppermost and lowesttriangular sweeps in FIG. 4 have a mutual time delay and frequency shiftfor example.

When a predetermined time delay between the frequency sweeps of thereceived signal and the local FMCW signal and a predetermined frequencyshift between the received signal and the local FMCW signal arerealized, this has the following effect. Initially first frequencydifference D1 occurs between the received signal and the local FMCWsignal, while the frequencies of both signals move in the samedirection. This difference gives rise to a first mixed down frequencyD1. The first frequency difference D1 equals the predetermined frequencyshift plus the product of the predetermined time delay and the sweeprate. Next a short transitory time interval occurs wherein the frequencydifference changes, when the direction of change of the frequency of onesignal has reversed and the other has not. After the transitory timeinterval a second first frequency difference D2 occurs, which gives riseto a second mixed down frequency D2. The difference D1-D2 between thefirst and second mixed down frequencies is equal to twice the frequencysweep rate times the time delay between the sweeps of the receivedsignal and the local FMCW signal. This difference is independent of thefrequency offset.

The relevant mixed down frequencies are non-zero in the synchronizedstate with the predetermined a time delay and frequency shift. Thisreduces the effect of low frequency noise. Mixer 3 may comprise a bandpass filter (IF filter) to filter the mixed down signal for input tocontroller 5, or band pass filters for the in phase and quadrature phasemixing products, if a quadrature mixer is used. Furthermore, therelation between the mixed down frequencies facilitates robust detectionof the received signal. The distance between the relevant frequenciesmakes it possible to eliminate other signals, which do not occur inpairs at this frequency distance.

The predetermined time delay may have a time value which corresponds(via the value of c) to e.g. 10 meters, which (predetermined) value isdiscounted in the calculation of the distance between terminals A and B.Such intentional deviation from zero has the advantage that theelectronic circuitry works better because reflections of objects in thedirect vicinity of terminals A and/or B will not or less disturb theinteroperation of the terminals.

Controller 5 may be configured to adjust the timing of FMCW generator 2until frequency peaks at a mutual distance of the frequency sweep ratetimes the predetermined time delay between the sweeps arise. In this waytiming of the received signal and the local FMCW signal may besynchronized with a controlled mutual delay.

If a later cycle of the predetermined local FMCW signal with apredetermined time delay is used to transmit an FMCW signal back toterminal A for distance determination, this will affect the distancedetermined by terminal A. But as the effect of the time delay is known(a distance correction of the time delay times the signal propagationspeed), terminal A may be designed to compensate for the effect. Whenterminal A determines the distance, terminal A may correct for the delayby subtracting the predetermined delay times the speed of light. Thefrequency of the local FMCW signal may be controlled based on afrequency reference in terminal, such as a crystal. If desired,controller 5 may be configured to adjust the frequency of FMCW generator2 based on the average frequency of the peaks, or the frequency of oneof the peaks so that a predetermined frequency shift is realized.Similarly, controller 5 may be configured to adapt the sweep rate basedon the detected signal.

The frequencies in the resulting mixed down signal may be determined bysampling the mixed down signal during a sweep period of the local FMCWsignal and computing the Fourier transform of the sampled signal.Preferably the sampled signal values are digitized using one or moreanalog to digital converters. Controller 5 may comprise an FFT modulefor computing the Fourier transform.

The resulting Fourier transform will exhibit amplitude peaks as afunction of frequency. Controller 5 may comprise a signal processingcircuit configured to detect these peaks and to determine their positionfor use to control synchronization. When a quadrature mixer is used, andin phase and quadrature phase outputs are used as real and imaginaryparts of the input numbers of the FFT, peaks at positive and negativefrequency arise independently. Otherwise it may be desirable to use afrequency offset so that the peaks occur both at positive or both atnegative frequency. Controller 5 may comprise a signal processingcircuit configured to detect these peaks and to determine theirposition, and a control module, configured to control FMCW generator 2to adjust the time delay of its local FMCW signal in proportion to adeviation between the distance between the detected peaks and a distancecorresponding to the predetermined time delay. In other embodiments thefrequencies before and after the transitory time interval may bedetermined separately, for example using separate FFT computations. Inother embodiments the frequency determination need not be linked to thesweep period of the local FMCW signal. It suffices that a signal isderived that is indicative of the difference between the frequencypositions of components in the mixed down signal.

In an embodiment, controller 5 may be configured to operate in a searchphase before adjusting the synchronization as described, when nofrequency peaks of sufficient strength are detected. When there is nosynchronization, the mixed down beat frequency between the receivedsignal and the local FMCW signal may lie outside the filter bandwidth ofmixer 3. In the search phase, controller 5 may adjust the timing andoptionally the frequency in steps until frequency peaks of sufficientstrength are detected.

FMCW generator 2 may be configured to sweep the local FMCW signal inperiodic cycles, each with an identical sweep, such as the triangularsweep. In this embodiment, controller 5 may be configured to switchterminal B to a transmit mode in a mute period of terminal A, the localFMCW signal being passed to the antenna via amplifier 6 in the transmitmode. Thus, a cycle of the local FMCW signal is transmitted after thelocal FMCW signal has been synchronized to the received signal in one ormore previous cycles. Terminal A may comprise a similar FMCW generator2, which also produces periodic cycles. Hence terminal A is enabled todetermine the distance by mixing the FMCW signal from terminal B withits own FMCW signal in the mute period.

In other embodiments periodic cycles need not be used. One transmittedsweep cycle of the FMCW signal from terminal B, or even only a part of acycle, suffices for distance determination. As long as controller 5 hasdetermined the local timing of the transmitted signal, it can triggersuch a cycle or partial cycle in a predetermined timing relationshipwith transmission by terminal A. In another embodiment controller 5 mayrecord the received FMCW signal, preferably after normalizing itsamplitude, and retransmit the recorded FMCW signal in a predeterminedtime relation to reception, for example after a delay time interval ofpredetermined length, or a selected one of a plurality of delay timeintervals that start at different predetermined time points relative toreception of the signal from terminal A, controller 5 selecting a delaytime interval that at least partly coincides with a mute period of thereceived signal. Although predetermined time shifts have been used,which allowed terminal A to compensate these time shifts frompredetermined information, it should be appreciated that datarepresenting the applied time shift could be transmitted to terminal Aand/or terminal B, terminal A using the transmitted data to apply thecompensation for the time shift to the distance calculation.

As noted, terminal A and terminal B may have identical components. Inthis case the difference lies in the selected operation mode. Thepreceding describes a mode wherein terminal A performs FMCW baseddistance measurement and terminal B transmits a FMCW in response. Theterminals could be switched to an opposite mode wherein terminal Bperforms FMCW based distance measurement and terminal A transmits a FMCWin response. The distance may be determined by determining the timedelay between the FMCW signal in terminal A and the received signal fromterminal B. In a further embodiment, the distance may be determined bygenerating a synchronized FMCW signal in terminal A, as described forterminal B, the controller of terminal A determining the distance fromthe time offset that needs to be applied to achieve the localsynchronized FMCW signal (e.g. the predetermined frequency distancebetween the detected frequency peaks), compared to the time shift, ofany, used to generate the original transmitted signal.

Although examples with sawtooth and triangular FMCW sweeps have beendescribed, it should be appreciated that other sweep shapes may be used,which enable determination of transmission delay from the time shift ofthe frequency to time dependence.

Summarizing, an embodiment of a method for measuring the distancebetween a terminal A and a terminal B is provided, wherein each terminalcomprises

-   -   a generator arranged for generating a signal having a        predetermined frequency to time function, called FMCW generator        and FMCW signal respectively hereinafter;    -   a transmitter for transmitting a transmit signal comprising at        least one cycle of the FMCW signal,    -   a receiver for receiving a signal transmitted by the other        terminal. During execution of the method terminal A transmits a        transmit signal, which is received by the receiver of terminal        B;    -   terminal B shifts the local signal and/or the received signal in        frequency and/or in time such that the local signal has a        predetermined frequency offset and/or time offset compared with        the received signal;    -   terminal B transmits the shifted signal, which is received by        the receiver of terminal A;    -   terminal A measures the frequency offset between its own        transmit signal and the signal received from terminal B and        calculates the distance between terminal A and terminal B from        that measured frequency offset, taking into account said        predetermined frequency and/or time offset performed in terminal        B.

In a further embodiment said predetermined frequency to time functionhas a triangular shape. In another embodiment said predeterminedfrequency offset is zero. In another embodiment said predeterminedfrequency offset is a value unequal to zero. In an embodiment thetransmit signal is constituted by alternating a transmit period,including n cycles of the signal generated by the FMCW generator, with amute period corresponding with n cycles of that signal, n being at least1.

Summarizing, an embodiment of a method for measuring the distancebetween a terminal A and a terminal B is provided, wherein each terminalcomprises

-   -   a generator arranged for generating a signal having a        predetermined frequency to time function, called FMCW generator        and FMCW signal respectively hereinafter;    -   a transmitter for transmitting a transmit signal comprising at        least one cycle of the FMCW signal,    -   a receiver for receiving a signal transmitted by the other        terminal. During execution of the method    -   terminal A transmits a transmit signal, which is received by the        receiver of terminal B;    -   terminal B shifts the local signal and/or the received signal in        frequency and/or in time such that the local signal has a        predetermined frequency offset and/or time offset compared with        the received signal;    -   terminal B transmits the shifted signal, which is received by        the receiver of terminal A;    -   terminal A measures the frequency offset between its own        transmit signal and the signal received from terminal B and        calculates the distance between terminal A and terminal B from        that measured frequency offset, taking into account said        predetermined frequency and/or time offset performed in terminal        B

In an embodiment the method comprises mixing the received transmitsignal with the local signal, and adjusting a timing of the local signaldependent on an output signal of the mixer during a period in which thetransmit signal is received and to enable transmission of the localsignal during a mute period. In a further embodiment the methodcomprises using a triangular frequency to time relation wherein thefrequency first changes in a first direction at a frequency sweep rateand subsequently changes in a second direction, opposite to the firstdirection at said frequency sweep rate, and detecting a frequencydifference between frequency components of the output signal of themixer and to adjust the timing of the local signal dependent on thefrequency difference.

In an embodiment a terminal is provided for use in a system wherein adistance between terminal (B) and a further terminal (A) is measuredfrom a transmission delay of a transmit signal having a predeterminedfrequency to time relation, the terminal (B) comprising

-   -   a receiver for receiving the transmit signal from the further        terminal;    -   a mixer,    -   a generator configured to generate a local signal having a        non-zero time offset compared to the received transmit signal,        the generator being coupled to the mixer, the mixer being        configured to mix the received transmit signal with the local        signal;    -   a controller with an input coupled to an output of the mixer and        a control output coupled to the generator, the controller being        configured to adjust a timing of the local signal dependent on        an output signal of the mixer during the transmit period and to        enable transmission of the local signal during the mute period.

In a further embodiment the transmit signal comprises a triangularfrequency to time relation wherein the frequency first changes in afirst direction at a frequency sweep rate and subsequently changes in asecond direction, opposite to the first direction at said frequencysweep rate, the controller being configured to detect a frequencydifference between frequency components of the output signal of themixer and to adjust the timing of the local signal dependent on thefrequency difference.

In the following an embodiment will be described wherein the transmittedsignal is also used for data transmission.

In the following, the following abbreviations may be used:

-   AM Amplitude Modulation-   ARTS Active Ranging Transponder System-   BPSK Binary Phase Shift Keying-   CW Continuous Wave (typically having a constant amplitude)-   D-PPM Differential Pulse Position Modulation-   D-QPSK Differential Quadrature Phase Shift Keying-   DFT Discrete Fourier Transform, often referred to as FFT-   FFT Fast Fourier Transform, actually implemented as DFT-   FM Frequency Modulation-   FMCW FM CW-   ISI Inter Symbol Interference-   NCO Numerically Controlled Oscillator-   OFDMOrthogonal Frequency Division Multiplex-   PPM Pulse Position Modulation-   QPSK Quadrature Phase Shift Keying-   RF Radio Frequency (14 kHz-300 THz)-   SNR Signal to Noise Ratio-   8-PSK 8 Phase Shift Keying-   16-QAM 16 Quadrature Amplitude Modulation

In the figures the following symbols are used:

-   BFS Beat frequency spectrum-   Ampl Amplitude-   BFS Beat Frequency from Synchronization sweep-   BFD Beat Frequency from Data sweep-   BF Beat Frequency-   fd the smallest beat frequency step-   BFDP Beat Frequency of Dominant Path-   SR Response from Synchronization sweep-   DR Response from Data symbol sweep-   SR Shift register-   AB Analysis block

After the synchronization step the terminal B may transmit data toterminal A by modulating the (previously synchronized) local signal bythe controller 5, under control of the data to be transmitted fromterminal B to terminal A. Several modulation methods are applicable,e.g. OFDM, PPM, D-PPM, PPM/AM and D-PPM/AM. By means of an FFT module in(the receiver part of) terminal A the OFDM, PPM, D-PPM, PPM/AM orD-PPM/AM modulated signal can be converted (demodulated) to data. Inthis way the same terminal sets may be used for distance measurement anddata transfer.

Hereinafter a number of applicable modulation methods are discussed morein detail, with reference to the FIGS. 5 a-i.

OFDM

Orthogonal frequency-division multiplexing (OFDM), essentially identicalto coded OFDM (COFDM) and discrete multi-tone modulation (DMT), is afrequency-division multiplexing (FDM) scheme utilized as a digitalmulti-carrier modulation method. A large number of closely-spacedorthogonal sub-carriers (“bins”) are used to carry data. The data isdivided into several parallel data streams or channels, one for eachsub-carrier. Each sub-carrier is modulated with a conventionalmodulation scheme (such as quadrature amplitude modulation orphase-shift keying) at a low symbol rate, maintaining total data ratessimilar to conventional single-carrier modulation schemes in the samebandwidth.

In good SNR situations, OFDM is certainly applicable. However it is tobe realized that in practical situations:

-   1) The OFDM subcarriers could be modulated BPSK, QPSK, 8-PSK, 16-QAM    etc. and received with low Bit-Error Rate, as long as the SNR is    sufficient.-   2) With OFDM with n subcarriers, the transmitter power has to be    divided among these carriers. This reduces the transmitter power per    subcarrier by a factor n, worsening link budget.-   3) A further lowering of power is dictated by the OFDM crest factor    (peak-to-average), forcing the analog chain to be operated at an    even lower power level, further worsening link budget.

Points 5 and 6 describe drawbacks of OFDM. However, as mentioned, ingood SNR cases OFDM works.

In an embodiment OFDM modulation may be superimposed on the sweep, forexample by mixing a base band OFDM signal with a swept signal. Inanother embodiments the sweep may be replaced by an OFDM signal.

PPM

Pulse-position modulation (PPM) is a form of signal modulation in whichM message bits are encoded by transmitting a single pulse in one of 2Mpossible time-shifts. This is repeated every T seconds, such that thetransmitted bit rate is M/T bits per second. It is primarily useful foroptical communications systems, where there tends to be little or nomultipath interference.

To overcome bad link budget situations PPM could be applied:

-   1) PPM provides substantially equal SNR as ranging-   2) PPM allows re-use of the RF and baseband circuitry used for    ranging.

In FIGS. 5 a and 5 b, illustrating the method according to theinvention, the smallest beat frequency step is represented by fd. Ashift can be made in differing magnitudes m*fd,

Let the receiver of terminal A contain an FFT. Let fd be the FFT binresolution. If there are 256 FFT bins and the receiver resolutionaccommodates this, one symbol could e.g. represent 8 bits. With 1 kHzsweep rate, 8 kb/s could be transmitted. In general, if there are n FFTbins, one symbol represents ² log n bits and the peak data rate is:

Rp=sweep rate*² log n.

D-PPM

One of the key difficulties of implementing PPM is that the receivermust be properly synchronized to align the local clock with thebeginning of each symbol. Therefore, PPM is often implemented asdifferential pulse-position modulation (D-PPM), wherein each pulseposition is encoded relative to the previous one, such that the receivermust only measure the difference in the arrival time of successivepulses. It is possible to limit the propagation of errors to adjacentsymbols, so that an error in measuring the differential delay of onepulse will affect only two symbols, instead of affecting all successivemeasurements. So in cases where the frequency error of receiver andtransmitter is an issue, an offset-independent scheme as D-PPM (compareDQPSK) can be used. This implies that not the absolute positions of thepeaks, but only the step-wise differences between the peaks are intendedto contain data. The advantage is that this method is tolerant to someloss of exact time-synchronization, thus allows relatively large amountsof data to be transferred without the necessity to synchronizeregularly, thereby reducing overhead.

PPM/AM

Where the SNR is sufficient, additional amplitude modulation can beapplied to (D-)PPM modulated FMCW signals, see FIG. 5 c.

Multipath

In case of multipath transmission, the received base band spectrum canhave a shape consisting of multiple peaks, as shown in FIG. 5 d.

It may be noted that the shape is the result of the channel impulseresponse. The shape depicted in this figure as an example, is differentfrom an actual impulse response, because it represents the appearancewhen a modulus operation is applied. After the FFT the beat frequencyspectrum is complex and may have to be mathematically treated as suchfor the most accurate result.

When data are transmitted using PPM or D-PPM, the entire pattern shiftsbut maintains its shape, see FIG. 5 e.

When no action taken, multipath transmission degrades reception at longrange: at long range the signal level is low, giving bad SNR and alsothe delay spread is larger, giving a further decrease of discrimination.These ISI (Inter Symbol Interference) effects are somewhat similar toeye pattern degradation in i.e. WiFi and optical transmission systems.

Multipath Remedies

Below a few methods to reduce intersymbol interference are described.These methods may be combined in some embodiments.

Multipath Remedy 1

Apply steps m*fd, which separates the responses but lowers the datarate, illustrated in FIG. 5 f.

Multipath Remedy 2

Apply equalizer-training by transmitting a number of sync sweeps. Thereceiver performs synchronization to the largest peak simultaneouslywhile averaging out the noise. This could be done either in the complexdomain, the modulus domain or the log modulus domain, depending onactual implementation constraints.

Multipath Remedy 3

The received pattern, from the FFT, is reduced to an impulse functionusing a correlator module. The correlator module sums a series of signalvalues, each multiplied by a respective tap factors. The tap factors ofthe correlator module are first determined by using an ‘inversion’algorithm. The inversion algorithm aims to deconvolute the effect ofmultipath transmission. A received signal that results from a knowntransmitted signal is measured, possibly averaged over a number ofcycles, and a set of tap factors is determined that minimizes a measureof difference between the known transmitted signal and the receivedsignal. Inversion algorithms are known from echo cancelling techniquesfor example.

As a result of correlation the modulus of the beat frequency spectrum isreduced to a single impulse function. In fact the method applied to themodulus of the beat frequency spectrum, is identical to a treatment, asif it were a discrete time function: A discrete time representation ofan impulse response can be correlated to its inverse resulting in animpulse function. The effect of equalizer training, the removal of ISI,can be interpreted and implemented as pattern recognition:

-   -   The averaged reference pattern, which is obtained on reception        of sync sweeps is stored;    -   The received FFT bin outputs are serialized and led along a        correlator;    -   The largest peak from the correlator module represents the data        symbol being received.

Instead of applying correlation to the FFT signal, it may be applied tothe time dependent signal, for example before applying the FFT.Correlator modules

Multipath Remedy 4

Apply a state of the art correlator module, e.g. having a topology asdepicted in FIG. 6. There are two consecutive sweeps. Receiving thefirst sweep, at the FFT output are n complex values w1 . . . wn. Theseare stored as a complex vector “A” in a parallel register “a”. Receivingthe second sweep, at the FFT output are again n complex values areavailable, s1 . . . sn, as a complex vector “B”. It may be convenient tostore B in a shift register “b”. FIG. 5 g shows the effect to vector Aafter FFT of impulse response of channel. FIG. 5 h shows the effect tovector B after FFT of impulse response of channel.

Next, a convolution is performed Conv(A,B*), by stepwise-shifting Bthrough the shift register b in 2n−1 consecutive steps, using thetopology. The 2n results are stored as complex vector “C” in a shiftregister. The vector “C” will contain vectors to be regarded as powers.An analysis block may perform a modulus operation to obtain vector |C|which will in most practical delay spread cases substantially contain 1symmetric peak. B has no time shift to A. The sole presence of whichaids in simplifying demodulation of the DPPM data.

Finally, FIG. 5 i depicts a situation in which B is transmitted later inthe frame than A, which translates into a peak which deviates an amountΔ from centre cn. Δ represents the relative time step in DPPM. |B has atime shift with respect to A.

It may be noted that the multipliers in the correlation topology performthe operation: z=x·y*, where y* represents the complex conjugate of y.

FIG. 7 shows a terminal B that is configured to perform bothsynchronization and data transmission. The terminal comprises an antenna70, a mixer 72, a controller 74, an FMCW generator 76, and a switch 78.Switch 78 may be an electronic switch, comprising one or moretransistors. Switch 78 is coupled between antenna 70 on one side and onthe other side to an input of mixer 72 and a first output of FMCWgenerator 76. The output or a second output of FMCW generator 76 iscoupled to a second input of mixer 72. Mixer 72 has an output coupled tocontroller 74. Controller 74 has a control output coupled to FMCWgenerator 76. Furthermore, controller 74 has a data input/output.Although not shown, it should be appreciated that additionally theterminal may comprise amplifiers, such as a antenna pre amplifier and anoutput amplifier, and additional intermediate mixing stages. Althoughsingle lines have been used to symbolize connections, it should beemphasized that in fact multiple conductors may be used for connections.For example, mixer 72 may be a quadrature mixer, having outputs for inphase and quadrature signal components.

FMCW generator 76 comprises sweep generator module 760, a modulatormodule 762, and a controllable oscillator 764. Sweep generator module760 has an output coupled to controllable oscillator 764. Controllableoscillator 764 has an output coupled to switch 78 and to mixer 72.Controller 74 comprises a filter 741, a sampling circuit 740, an FFTmodule 742, an optional correlator module 743, a frequency detectormodule 744, a control signal generator 746 and data module 748. Samplingcircuit 740 has an input coupled to the output of mixer 72 via filter741 and an output coupled to FFT module 742. Sampling circuit 740 maycomprise an ADC (analog to digital converter). When quadrature signalsare used, filter 741 may comprise filter components for in phase andquadrature signals and sampling circuit 740 may be configured to sampleboth signal components. FFT module 742 has an output coupled tocorrelator module 743. Frequency detector module 744 has an inputcoupled to an output of correlator module 743 and an output coupled tocontrol signal generator 746. Control signal generator 746 has outputscoupled to switch 78, controllable oscillator 764 and to sweep generatormodule 760 via modulator module 762. Data module 748 is coupled to thedata input/output of controller 74. Data module 748 has an input coupledto the output of frequency detector module 744 and an output coupled toa control input of modulator module 762.

Part or all of the module of controller 74 and FMCW generator 76 may beimplemented as digital signal processing modules. In this case a digitalsignal processing computer may perform the functions of the variousmodules under control of stored instructions from program modules forrespective functions. Alternatively, part or all of the modules ofcontroller 74 and FMCW generator 76 may be implemented as discretecircuits, on one or more integrated circuits for example.

In operation, controllable oscillator 764 generates a high frequencysignal with a frequency controlled mainly by sweep generator module 760.Control signal generator 746 controls switch 78 to switch between areceive state wherein signals from antenna 70 are passed to mixer 72 anda transmit state wherein signals from controllable oscillator 764 arepassed to antenna 70. In the receive state, mixer 72 mixes down thereceived signal from antenna 70 using the signal from controllableoscillator 764. When controllable oscillator 764 is at least roughlysynchronized with the received signal, this results in a low frequencysignal from mixer, which is filtered and sampled. FFT module 742computes a Fourier Transform from this signal. Correlator module 743reduces the effects of multipath transmission and frequency detectormodule 744 detects the position of frequency peaks in the Fouriertransformed signal.

Control signal generator 746 cyclically selects a sweep control signaldependent on the position of frequency peaks. The sweep control signalcontrols the starting time points of the sweep generated by sweepgenerator module 760, so that they are synchronized with time pointsdefined by the received signal (allowing for an offset). Because theposition of frequency peaks is used for this, information from theentire sweep cycle is used to determine the latter time points.

Form the starting time points control signal generator 746 determinesthe time intervals of the sweeps that will be transmitted back. At thestart and end of these time intervals control signal generator 746switches control switch 78 to the transmit state and the receive staterespectively. Control signal generator 746 does not use the position ofthe frequency peaks in these time intervals to update the sweep controlsignal.

When information has to be sent back, data module 748 causes modulatormodule 762 to shift the starting time points of the sweep in a datadependent way. Different amounts of shift may be used for different datasymbols for example. Data module 748 may also use the detected positionof the frequency peaks to decode data from the received signal. In anembodiment this is done in selected sweep cycles, control signalgenerator 746 not using the position of the frequency peaks in thesetime intervals to update the sweep control signal. In anotherembodiment, data module 748 selects the amount of shift dependent on thedata and causes modulator module 762 to apply the selected amount ofshift in one direction and in the opposite direction in respective sweepcycles. This enables terminal A to remove the effect of the modulationon the determination of the distance, by averaging the shifts of itsreceived signal in these sweep cycles.

Although FIG. 7 illustrates one type of modulation, using shifts of thestarting time point of the sweep, it should be appreciated thatdifferent types of modulation may be used. In this case data module 748may also control the operation of controllable oscillator 764 to applyfrequency, phase and/or amplitude modulation during the sweep. When thereceived signal comprises OFDM modulation for example, data module 748may use the output of the FFT module 742 to derive the modulated data.In this way, FFT module 742 may be used both for synchronization anddemodulation.

Controllable oscillator 764, with its swept frequency, may be replacedby a combination of a not-swept oscillator, a lower frequencycontrollable oscillator and a mixer that mixes the signal of thenot-swept oscillator with that of the lower frequency controllableoscillator. The controllable oscillator may be a synthesizer circuitthat synthesizes the oscillator signal. When no data transmission isneeded the modulator module may be omitted.

In an embodiment, a type of modulation is used that maintains a zeroaverage frequency shift during a sweep cycle. Each data symbol may beencoded by a using a pair of modulation symbol with mutually oppositeeffect on the frequency shift for example. Thus, the effect ofmodulation on distance measurement may be minimized. In an embodiment,distinct modulation periods and distance measurement periods are used,no modulation being applied to the transmitted local FMCW in thedistance measurement cycles. In another embodiment, the controller 74may be configured to use the demodulated data to counteract the effectof modulation on the received signal before using the received signal tocontrol generation of the local FMCW signal.

It may be noted that the modulation and demodulation could also be usedwithout determining distance between the terminals, even in a terminalwithout modules for doing so. In this case the FMCW sweep provides forincreased robustness of transmission rather than for distancedetermination.

Summarizing, a method for transmitting data from a terminal B to aterminal A, may be used, wherein each terminal comprising a generatorarranged for generating a respective local signal having a predeterminedfrequency to time function, as well as a transmitter part fortransmitting a signal comprising at least one cycle of the local signal,and a receiver part for receiving a signal transmitted by the otherterminal. The method preferably comprises the following steps:

-   a. terminal A transmits a signal A2B, which is received by the    receiver part of terminal B;-   b. terminal B synchronizes its local signal such that the local    signal and the received signal A2B have a predetermined time offset    relative to each other;-   c. terminal B modulates its local signal by shifting the local    signal in frequency and/or in time, under control of the data to be    transmitted from terminal B to terminal A;-   d. terminal B transmits its local signal, synchronized in step b and    modulated in step c, as a signal B2A_(data), which is received by    the receiver part of terminal A.

So in this method terminal B first shifts its local signal in time suchthat the local signal and the received signal A2B have a predeterminedtime offset relative to each other. After the synchronization step b theterminal B is ready to transmit data to terminal A, viz. by modulatingthe (shifted) local signal by shifting the local signal in frequencyand/or in time, under control of the data to be transmitted fromterminal B to terminal A.

This can be combined with distance measurement, when

-   b1. terminal B transmits its local signal, synchronized in step b.,    as a signal B2A_(sync), which is received by the receiver of    terminal A;-   b2. terminal A measures the frequency offset between its own local    signal and the signal received from terminal B and calculates the    distance between terminal A and terminal B from that measured    frequency offset, taking into account said predetermined frequency    and/or time offset performed in terminal B.

So after the synchronization step b. the terminal B is ready to transmita signal B2A_(sync) to terminal A, after which terminal A measures thefrequency offset between its own local signal and the signal receivedfrom terminal B and calculates the distance between terminal A andterminal B from that measured frequency offset.

Preferably, said predetermined frequency to time function has atriangular shape. That is, the frequency first increases linearly withtime and then decreases linearly with time. Other shapes of thefrequency to time function are also possible, such as a sawtooth shapewhere the frequency first increases linearly with time and thendecreases almost instantaneously to the start value.

Preferably, said predetermined frequency offset is zero.

Preferably, in terminal B the synchronized local signal is modulated bythe data using OFDM, PPM, D-PPM, PPM/AM or D-PPM/AM to form said signalB2A_(data).

The signals may be transmitted between the terminals by wire but arepreferably transmitted wirelessly. If wireless transmission is used, thefrequencies used are preferably radar frequencies, for examplefrequencies in the MHz (megahertz) or GHz (gigahertz) range, althoughthe present invention is not so limited.

A computer program product is provided, with a program of instructionsfor a programmable computer, for making the computer carry out themethod as defined above.

A computer program product may comprise a set of computer executableinstructions stored on a data carrier, such as a CD or a DVD. Theprogram of computer executable instructions, which allow a programmablecomputer to carry out the method as defined above, may also be availablefor downloading from a remote server, for example via the Internet.

The present invention additionally provides a terminal suitable forcarrying out the method of the present invention, as well as a systemarranged for carrying out the method of the present invention. Aterminal according to the present invention may comprise a generator forgenerating a local signal, a mixer for adding a received and a locallygenerated signal, and a switch for feeding the received signal from anantenna to the mixer. The terminal may further comprise a controller forcontrolling the generator and the switch (4), and may preferably furthercomprise an amplifier for feeding a signal to an antenna. In anadvantageous embodiment, the terminal further comprising an FFT unit fordemodulating a modulated received signal.

A system according to the present invention may comprise at least twoterminals as defined above, as well as antennas connected to therespective terminals.

In other words, summarizing this aspect, a method for transmitting datafrom a first terminal B to a second terminal A is provided, eachterminal comprising

-   -   a generator arranged for generating a respective local signal        having a predetermined frequency to time function,    -   a transmitter part for transmitting a signal comprising at least        one cycle of the FMCW signal,    -   a receiver part for receiving a signal transmitted by the other        terminal;

The method comprises the following steps:

-   a. terminal A transmits a signal A2B, which is received by the    receiver part of terminal B;-   b. terminal B synchronizes its local signal by shifting its local    signal in frequency and/or in time such that the local signal and    the received signal A2B have a predetermined frequency offset and/or    time offset to another;-   c. terminal B modulates its local signal by shifting the local    signal in frequency and/or in time, under control of the data to be    transmitted from terminal B to terminal A;-   d. terminal B transmits its local signal, synchronized in step b and    modulated in step c, as a signal B2A_(data), which is received by    the receiver part of terminal A.

In an embodiment between steps b. and c.,

b₁. terminal B transmits its local signal, synchronized in step b, as asignal B2A_(sync), which is received by the receiver part of terminal A;b₂. terminal A measures the frequency offset between its own localsignal and the signal received from terminal B and calculates thedistance between terminal A and terminal B from that measured frequencyoffset, taking into account said predetermined frequency and/or timeoffset performed in terminal B.

The predetermined frequency to time function may have a triangularshape. Said predetermined frequency offset may be zero.

In step c., in terminal B the synchronized local signal is modulated bythe data using OFDM, PPM, D-PPM, PPM/AM or D-PPM/AM to form said signalB2A_(data).

A terminal (B; A) is provided for transmitting data in accordance withthe method according to any of the preceding claims. In an embodimentthe terminal, comprises a generator (2) for generating a local signal, amixer (3) for adding a received and a locally generated signal, a switch(4) for feeding the received signal from an antenna (1) to the mixer(3).

In a further embodiment the terminal further comprises a controller (5)for controlling the generator (2) and the switch (4), the terminalpreferably further comprising an amplifier (6) for feeding a signal toan antenna (1).

In a further embodiment the controller (5) is configured for detecting afrequency difference or time difference between a received and a locallygenerated signal, and preferably is configured for shifting, in aniterative process, a local oscillator signal in frequency and/or in timesuch that the local signal has a predetermined frequency.

In a further embodiment the terminal further comprises an FFT unit fordemodulating a modulated received signal.

It is noted that any terms used in this document should not be construedso as to limit the scope of the present invention. In particular, thewords “comprise(s)” and “comprising” are not meant to exclude anyelements not specifically stated. Single (circuit) elements may besubstituted with multiple (circuit) elements or with their equivalents.

It will be understood by those skilled in the art that the presentinvention is not limited to the embodiments illustrated above and thatmany modifications and additions may be made without departing from thescope of the invention as defined in the appending claims.

1. A method of measuring the distance between a first terminal and asecond terminal, the method comprising the following steps: transmittinga transmit signal from the first terminal in transmit periods, thetransmit periods alternating with mute periods, the transmit signalhaving a predetermined frequency to time relation; receiving thetransmit signal with a receiver of the second terminal; generating ashifted signal in the second terminal having a non-zero time offsetcompared to the received transmit signal and/or compared to a localsignal obtained using the received transmit signal, the time offsetshifting the shifted signal into one of the mute periods; transmittingthe shifted signal from the second terminal, receiving the shiftedsignal with a receiver of the first terminal; measuring a frequencyoffset between the transmit signal and the received shifted signal inthe first terminal; and calculating the distance between the firstterminal and the second terminal from the measured frequency offset,taking into account said time offset provided by the second terminal(B).
 2. A method according to claim 1, wherein the transmit signal has acyclical frequency to time relation in the transmit periods, including ncycles of the transmit signal, n being greater than one, the mute periodhaving a duration of at least one cycle duration.
 3. A method accordingto claim 1, wherein the shifted signal is generated by generating aperiodic signal in the second terminal, the periodic signal having saidpredetermined frequency to time relation in successive cycles, detectinga timing difference between the periodic signal and the received signalin the transmit period in the second terminal, determining an adaptionof a timing of the cycles of the periodic signal from the detectedtiming difference, and deriving the shifted signal from the periodicsignal in the mute period, with said adaptation.
 4. A method accordingto claim 1, comprising generating the local signal in the secondterminal, synchronizing the local signal to the received transmitsignal, at a predetermined time delay compared to the received transmitsignal, the first terminal taking into account said predetermined timedelay in the calculation of the distance.
 5. A method according to claim1 wherein the first terminal and the second terminal are tags.
 6. Aterminal for use in a system wherein a distance between terminal and afurther terminal is measured from a transmission delay of a transmitsignal having a predetermined frequency to time relation, wherein thefurther terminal transmits the transmit signal in transmit periods thatalternate with mute periods, the terminal comprising a receiver forreceiving the transmit signal from the further terminal; a generatorconfigured to generate a shifted signal having a non-zero time offsetcompared to the received transmit signal and/or a local signal obtainedusing the received transmit signal, the time offset shifting the shiftedsignal into a mute period in the transmit signal; a transmitter fortransmitting the shifted signal from the terminal.
 7. A terminalaccording to claim 6, wherein the generator is configured to generate atime shifted copy of at least a part of the received transmit signal. 8.A terminal according to claim 1, the terminal comprising a mixer, thegenerator having an output coupled to the mixer, the mixer beingconfigured to mix the received transmit signal with the local signal, acontroller with an input coupled to an output of the mixer and a controloutput coupled to the generator, the controller being configured toadjust a timing of the local signal dependent on an output signal of themixer during the transmit period and to enable transmission of the localsignal during the mute period.
 9. A terminal according to claim 8,wherein the transmit signal provides a triangular relation betweenfrequency and time, the being generator configured to generate the localsignal with a first signal part wherein the frequency changes in a firstdirection at a frequency sweep rate and subsequently a second signalpart wherein the local signal changes in a second direction, opposite tothe first direction, at said frequency sweep rate, the controller beingconfigured to detect a frequency difference between frequency componentsof the output signal of the mixer due to mixing during the first andsecond signal part, and to adjust the timing of the local signaldependent on the frequency difference.
 10. A terminal according to claim9, wherein the controller is configured to set the frequency differenceto a predetermined value by adjust the timing.
 11. A terminal accordingto claim 1, wherein the controller is configured to switch between atransponder mode, wherein the local signal is used to generate theshifted signal and a distance measuring mode, wherein the local signalis transmitted to the further terminal or another terminal, forobtaining a return signal, the controller being configured to measure afrequency offset between its local signal and the return signal and tocalculate the distance between the terminal and the further terminal oranother terminal from the measured frequency offset.
 12. A terminalaccording to claim 11, the terminal comprising a mixer, the generatorbeing coupled to the mixer, the mixer being configured to mix thereceived transmit signal with the local signal, a controller with aninput coupled to an output of the mixer and a control output coupled tothe generator, the controller being configured to make an adjustment ofa timing of the local signal dependent on an output signal of the mixerduring reception of the return signal and to measure the frequencyoffset between its local signal and the return signal from theadjustment.
 13. A terminal according to claim 1, comprising a data inputand a modulator configured to modulate the local signal dependent ondata received from the data input.
 14. A tag comprising a terminalaccording to claim
 1. 15. A network comprising a plurality of terminalsaccording to according to claim
 1. 16. A computer program productcomprising a program of instructions for a programmable processorcircuit that, when executed by the programmable processor circuit causesthe programmable processor circuit to receiving a transmitted FMCWsignal from a further terminal; generate a shifted FMCW signal having anon-zero time offset compared to the received transmit signal and/or alocal signal obtained using the received transmit signal, the timeoffset shifting the shifted signal into a mute period in the transmitsignal; cause the shifted FMCW signal to be transmitted.